Geological Societi/. 557 



The gault of this district has been cut through to the depth of 

 120 feet, at Alice Holt, and iridescent Ammonites and other 

 fossils are found in it. This clay is marked by fertile water- 

 meadows, and the timber presenting a green belt clearly distin- 

 guishes it from the rich wheat land of the malm rock above, and 

 the arid expanse of the ferruginous green sand below it. 



Of this latter formation, the upper beds consist of pure white 

 sand, and in some places compact ironstone, and ironstone in large 

 cellular tubes are found. In the middle beds occurs a calcareo- 

 siliceous grit, called Bargate stone ; in the lower, a siliceous yellow 

 building stone, containing casts of Ammonites, Terebratulce^ &c. 

 The Weald clay includes in its middle beds the compact Petworth 

 marble, and in lower beds of clay, in which tabular calcareous 

 grit occurs, Mr. Murchison has discovered, together with scattered 

 shells of the Vivipara Jluviorum^ the bones of a large unknown 

 vertebrated animal, specimens and drawings of which accompany 

 this memoir. 



January 20, 1826. — The reading of a paper was concluded, 

 " On the Geology of Jamaica; by H. T. De la Beche, Esq. F.R.S. 

 F.G.S., &c." The following is an abstract of a portion of Mr. 

 De la Beche's communication, relating to a tertiary formation in 

 Jamaica, and the organic remains it includes. 



Trap rocks, consisting of porphyritic conglomerate, porphyry, 

 greenstone, and syenite, shew themselves very extensively in 

 Jamaica, composing the greater part of the St. John's mountains, 

 and the district bordering on the Agua Alta. These trap rocks 

 are found, generally, supporting the Great White - Limestone 

 formation^ which occupies a very large portion of the whole 

 island. This formation, from the fossils it contains, is referred 

 by Mr. De la Beche to the Tertiary series. It is principally com- 

 posed of white limestone, most frequeutly very compact, and then 

 strongly resembling the compact varieties of the Jura limestone. 

 The strata are usually very thick, varying from three to twenty 

 feet in breadth. In some districts, this rock is interstratified with 

 thick beds of red marie, and sandstone, and white chalky marie. 

 The compact limestone constitutes the middle part of the forma- 

 tion : the lower beds consist, chiefly, of sands and marles, some- 



